Power transmissions having planetary gear arrangements include selectively engageable friction devices that are operable to provide a number of gear ratios, thereby providing more efficient employment of a prime mover having a limited operating speed range. An electro-hydraulic system is utilized to control the interchange of ratios in the planetary gearing.
The ratios are interchanged by engaging one friction device, such as a clutch or brake, and disengaging another friction device, such as a clutch or brake. During a torque phase portion of the interchange period, both friction devices are at least partially engaged. The off-going device is held fully engaged until the on-coming device has a predetermined torque capacity.
The point at which the off-going device should begin to release can be difficult to determine. Some prior art control systems have used the torque level at the output shaft as exemplified by stress therein.
Other systems have used a "washout" shift, wherein a band brake is the off-going device and the servo piston thereof has an area available to be pressurized, by the fluid feed to the on-coming device, to enforce disengagement of the brake.
Still other systems use a time phased control which is adjusted by the harshness or softness of the previous shift for a given set of input parameters. The most popular method of controlling upshift timing is the use of a one-way torque transmitting device, either alone or in series, with a friction device.